Hi.
Is it possible now?
Can u usw a longsword in your main hand and ur second hand is empty to get 1w8 and not 1w10.
Sorry my English is not perfect.

In D&D5, 'Versatile' means that the weapon can be wielded one-handed or two-handed. The two-handed wield option inflicts more damage.
BG3 has, so far as I can see (I've not tested every combination), implemented this, so the answer is yes.